At present, researches on Ziegler-Natta catalysts involve the control of catalytic activity, studies on the behavior of polymerization, the effects of catalyst properties on the polymer properties, and so on. Among these researches, the enhancement of polymerization efficiency of catalysts is still one of important aspects for improving the performances of catalysts. Generally, although a chain transfer agent can be used for adjusting molecular weight of polymer in a polymerization process, it may affect the activity of catalyst in some extent simultaneously. A usually used chain transfer agent is hydrogen gas, and molecular weight of polymer is generally adjusted by adjusting a level of hydrogen in a polymerization process. However, with the increase of hydrogen content, polymerization activity of a catalyst typically decreases remarkably.
For example, the Chinese Patent ZL98110608 discloses a supported catalyst for gas-phase polymerization of ethylene. When the polymerization is carried out at a polymerization pressure of 1.0 MPa and an ethylene fractional pressure of 0.5-0.6 MPa for 4 hours, said catalyst exhibits a polymerization activity of from 2000 to 5000 grams polyethylene per gram catalyst. However, as shown in gas-phase polymerization evaluation carried out in Example 10 of said patent, when hydrogen level increases from H2/C2H4=0.125 to H2/C2H4=2.0, the polymerization activity of the same catalyst deceases from 5020 g polyethylene/g catalyst to 2500 g polyethylene/g catalyst, i.e., drops by about 50%, while the melt index of polyethylene (MI2.16) increases 3.9 g/10 min to 102 g/10 min.
The Chinese Patent ZL98110609 discloses a catalyst for slurry polymerization of ethylene. In slurry polymerization evaluation carried out in Example 10 of said patent, when hydrogen level in the polymerization system increases from H2/C2H4=0.33 to H2/C2H4=3.0, polymerization activity of a catalyst decreases from 459,000 g polyethylene/g Ti to 58,000 g polyethylene/g Ti, i.e., drops by about 85%, while melt index of polyethylene (MI2.16) increases from 3.4 g/10 min to 322 g/10 min. Thus, there is a need to develop a catalyst exhibiting good balance between activity when preparing high molecular weight (low melt index) polymer and that when preparing low molecular weight (high melt index) polymer.
U.S. Pat. No. 5,770,540 and EP 0949274 separately disclose a catalyst for polymerization of ethylene, which exhibits a good activity balance when preparing high molecular weight (low melt index) polymer and when preparing low molecular weight (high melt index) polymer. Said catalyst is prepared by reacting an inorganic support, a chloride supported on said support, a magnesium compound and a titanium halide, wherein the magnesium compound is obtained by reacting a dialkyl magnesium with same or different alkyl groups and a monohydric alcohol with a branched chain. Said catalyst exhibits a high polymerization activity, and the polymerization activity is not sensitive to change of hydrogen level in the polymerization system. In the polymerization examples of U.S. Pat. No. 5,770,540, when the hydrogen level increases from H2/C2H4=0.25 to H2/C2H4=2.0, polymerization activity of a catalyst decreases from 6000 g polyethylene/g catalyst to 4200 g polyethylene/g catalyst, i.e., drops by merely 30%, while melt index of polyethylene (MI2.16) increases from 2.0 g/10 min to 140 g/10 min. However, said catalyst must be prepared by using a dialkyl magnesium as starting material, and thus has a relatively high cost.